Dilute combustion of gasoline in an internal combustion engine, using either air or recirculated exhaust gas, can enhance the thermal efficiency and decrease the production of oxides of Nitrogen (NOx). However, there is a limit to which an internal combustion engine may operate with a dilute mixture due to misfire and combustion instability resulting from a slow burn rate of the dilute mixture. Known methods to extend the dilution tolerance limit include: 1) improving the ignitability of the mixture by enhancing ignition and mixture preparation, 2) increasing the flame speed by introducing charge motion and turbulence, and 3) operating the engine in a controlled auto-ignition combustion mode.
The controlled auto-ignition process may be referred to as Homogeneous Charge Compression Ignition (HCCI). In this process, a charge mixture of combusted gases, air, and fuel is created and auto-ignition is initiated simultaneously from multiple ignition sites within the compressed charge mixture, thereby resulting in stable power output and high thermal efficiency. Since the combustion is highly dilute and uniformly distributed throughout the charge mixture, the temperature of the burnt gas is typically lower than that of a traditional spark ignited engine with a propagating flame front and a diesel engine with an attached diffusion flame. The reduced temperature of the burnt gas may result in reduced NOx emissions when operating in the HCCI mode.
Four stroke gasoline internal combustion engines may operate in a controlled auto-ignition combustion mode by employing various valve opening and closing strategies. By altering the operating characteristics of the exhaust valves and/or the intake valves, a high proportion of residual burnt gases or products of combustion may be retained within the cylinder of the internal combustion engine to provide favorable conditions to auto-ignite a highly dilute charge mixture. The range of engine speed and load over which controlled auto-ignition combustion can occur may be expanded by employing various valve operating strategies, thereby obviating the need to increase the compression ratio of the spark ignited internal combustion engine.
One such valve operating strategy is exhaust re-compression. In this mode of operation, the exhaust valve is closed earlier in the exhaust stroke than in a typical four-stroke internal combustion engine. Correspondingly, the intake valve is opened later than in a typical four stroke internal combustion engine. The early exhaust valve closing and late intake valve opening provides a negative valve overlap period where products of combustion become trapped within the engine's cylinder. These trapped products of combustion will mix with the inducted fuel and air charge mixture during the intake stroke of the engine, thereby promoting the auto-ignition process.
Another valve strategy is exhaust re-breathing. In this mode, the exhaust valve is opened for a first period to allow combusted gasses to be expelled from the combustion chamber. Subsequently, the exhaust valve opens for a second period to allow products of combustion previously exhausted from the cylinder to be drawn back into the cylinder. By opening the exhaust valve twice during each four-stroke cycle of the internal combustion engine, the requisite conditions are created within the combustion chamber to support stable auto-ignition combustion.
Yet another valve strategy is a hybrid between exhaust re-compression and exhaust re-breathing. In this mode, the exhaust re-compression mode may be used when the engine is operating at a low engine load. For higher engine loads, the exhaust re-breathing strategy may be used. Additionally, by varying the exhaust valve lift and intake valve phasing, the spark-ignited engine may operate in a non-throttled load control mode (NTLC). In this mode, the intake valve phasing will vary the engine load by controlling the amount of air inducted into the cylinder. At the highest loads, the engine may operate in a traditional spark ignited fashion to enable maximum power density.
To enable the above-mentioned exhaust re-breathing valve strategy, the internal combustion engine may employ cam phasers, a two-step cam system, and a valve re-opening system.